LONDON (Reuters) - Global investors cut U.S. equity exposure in March on growing doubts about the "Trumpflation" rally, and slashed UK stock holdings to 5-1/2-year lows over worries about Brexit and a possible break-up of the United Kingdom.

A Reuters monthly asset allocation poll of 46 fund managers and chief investment officers in Europe, the United States, Britain and Japan showed investors trimming holdings of U.S. stocks to 40.8 percent of their global equity portfolios, from 41.2 percent in February.

This is the lowest level since Donald Trump was elected U.S. president in November on a platform of tax cuts and spending.

Investors have been piling into U.S. stocks, betting on a growth surge fueled by the promised fiscal splurge but Trump's recent failure to get a healthcare reform through Congress has caused the so-called "Trumpflation" rally to stumble.

The S&P 500 .SPX has fallen to six-week lows since Trump failed to push through that reform and looks set to end March flat after performing strongly in the previous four months.

"It might be that 'Trumponomics' is beginning to lose its effect as the president has talked the talk but as yet has failed to walk the walk," UK-based wealth manager Investment Quorum's chief investment officer, Peter Lowman, said.

Cash levels rose to 5.6 percent, the highest since November, with some managers expressing concern about asset prices, especially given political risks.

Robeco strategist Peter van der Welle said he had not changed his neutral stance on equities: "We feel reluctant at this point to chase the market, especially now cracks in the Trump trade become apparent."

UK BREAK-UP RISK

Asset managers also cut their UK equity exposure by 1.6 percentage points to 9.1 percent of global equity portfolios, the lowest level since September 2011, as Britain formally triggered the process to leave the European Union (EU).

The UK economy is likely to be severely tested during the two-year long process, with Scotland renewing its bid for an independence referendum. In the nine months since the Brexit vote, sterling has fallen about 17 percent against the dollar.

Although investors said it was too difficult to assess the impact of Brexit, 75 percent of those who answered an extra question on Scottish secession said the break-up of the United Kingdom would make them cut their UK exposure further.

"A Scottish referendum and secession would complicate significantly the internal political backdrop in the UK and would weaken the negotiating stance with EU, making it harder to achieve a constructive trade deal," Pioneer Investments' global head of multi-asset investments, Matteo Germano, said.

"Also it would lengthen the time to negotiate a deal and extend uncertainty."

Investors remained relatively bullish on euro zone equities, raising their exposure 1.6 percentage points to 18.6 percent of their global equity portfolios, the highest since August 2016.

European equities .FTEU3 are up about 2.4 percent for the month and 4.6 percent for the quarter, with Germany's DAX .GDAXI nearing an all-time high.

ECB UNWINDING

Some investors have revised their expectations about the timing of the ECB's unwinding of its ultra-loose monetary policy and, following weak inflation prints in the euro zone, many now see this happening later than previously anticipated.

Some 54 percent of poll participants who answered a specific question on ECB policy expect the bank to raise rates after 2018, and the remainder opted for 2018.

"We don't see an interest rate hike any time soon; the inflation projections, even if on the rise, remain mild," Pioneer's Germano said. "Interest rate hikes could trigger euro appreciation and an unwelcome tightening of financial conditions in the euro zone."

Robeco's van der Welle said that the ECB still faced challenges in a "two-speed euro zone economy confronted with many political roadblocks".

Meanwhile, some 62 percent of poll participants expect the U.S. Federal Reserve to raise rates three times in 2017, while about a third opted for two rate increases.

Royal London Asset Management's Head of Multi-Asset, Trevor Greetham, who opted for three hikes, noted the Fed's 25 basis point rate rise in March followed stronger labor market data, with the economy achieving greater momentum in recent months.

But he said, "With Trump's support in Congress uncertain and the scale, mix and timing of any fiscal stimulus still unclear, we expect the Fed to continue with a cautious approach to tightening."